astronomy

Magnetic starter

Magnetic starter is a astronomy topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Magnetic starter rather than just read about it. In short: A magnetic starter is an electromagnetically operated switch which provides a safe method for starting an electric motor with a large load. Magnetic starters also provide under-voltage and overload protection and an automatic cutoff in the event of a power failure.

Key takeaways

Magnetic starter belongs to astronomy; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Magnetic starter to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Magnetic starter from memory before moving on to harder problems.

Reference excerpt

A magnetic starter is an electromagnetically operated switch which provides a safe method for starting an electric motor with a large load. Magnetic starters also provide under-voltage and overload protection and an automatic cutoff in the event of a power failure.

Implementation A magnetic starter has a contactor and an overload relay, which will open the control voltage to the starter coil if it detects an overload on a motor. The overload relay opens a set of contacts that are wired in series with the supply to the contactor feeding the motor. The characteristics of the heaters can be matched to the motor so that the motor is protected against overload. Recently, microprocessor-controlled motor digital protective relays offer more comprehensive protection of motors. Due to the electromagnet in the contactor, if power to the machine should fail the contactor will automatically disengage. Unlike machines with an ordinary latching switch (such as a common light switch), when the power is resumed the machine will not operate until being turned on again. As a result, magnetic starters often use momentary switches for "off" and "on" functions, as this type of switch returns to a defined normal position when released. Latching switches do not have this feature and therefore are normally not used with a magnetic starter. Motor control contactors can be fitted with short-circuit protection (fuses or circuit breakers), disconnecting means, overload relays and an enclosure to make a combination starter. Several combination starters and other switchgear and control devices can be grouped in a common enclosure called a motor control center.

Operation Typically starters are operated by two push switches, a "start" switch which is normally-off (i.e. push-to-make) and a "stop" switch which is normally-on (i.e. push-to-break). When the motor is not running, although line voltage is available, no current is drawn by the starter or motor. When the "start" button is pressed, the motor is not powered directly, rather the electromagnet in the contactor is energized. The magnetic switch in the contactor then engages, simultaneously switching current to the motor and providing self-sustaining current to maintain its own state. Thus when the start button is released, the magnetic switch remains engaged and the motor remains running. Pressing the "stop" button breaks the circuit to the contactor which consequently de-energizes its electromagnet thus cutting current to the motor.

Applications Magnetic starters are commonly found on equipment drawing several horsepower or higher. Examples include woodworking machinery such as cabinet saws or shapers. Machines with smaller loads, such as a drill press or most handheld tools normally use only a switch instead. Magnetic starters are stock components for many machines, and aftermarket starters are also available for use as replacements or for retrofitting older machines. Federal OSHA recognizes NFPA 79 2007 Edition standard 7.5.3 states "Upon restoration of the voltage or upon switching on the incoming supply, automatic or unintentional restarting of the machine shall be prevented when such a restart causes a hazardous condition.

References

Illustrations

Worked examples

Example 1 — a first encounter with Magnetic starter

Start with the simplest possible case. Write down what Magnetic starter claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In astronomy, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Magnetic starter before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Magnetic starter ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Magnetic starter

In research: Magnetic starter appears in astronomy research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Magnetic starter in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Magnetic starter is common in secondary-school and first-year university syllabi. It links to neighbouring topics Over-current protection devices, Switches, so understanding it makes those chapters shorter.
In everyday life: Look for Magnetic starter outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Magnetic starter in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Magnetic starter means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Magnetic starter out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Magnetic starter in simple terms?

Why does Magnetic starter matter?

Because it connects several astronomy ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Magnetic starter?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Magnetic starter.